1. Technical Field
The invention relates to methods and apparatus for recovering samples of reservoir fluid.
2. Background of the Related Art
A reservoir is a rock formation in which fluids such as hydrocarbons, e.g., oil and natural gas, and water have accumulated. Due to gravitational forces, the fluids in the reservoir are segregated according to their densities, with the lighter fluid towards the top of the reservoir and the heavier fluid towards the bottom of the reservoir. One of the main objectives of formation testing is to obtain representative samples of the reservoir fluid. Commonly, reservoir fluid is sampled using a formation tester, such as the Modular Formation Dynamics Tester™ (MDT™), available from Schlumberger Technology Corporation, Houston, Tex. In practice, the formation tester is conveyed, generally on the end of a wireline, to a desired depth in a borehole drilled through the formation. The formation tester includes an inlet device, which may be a probe or packer, that can be set against the borehole wall and through which reservoir fluid can be drawn into a flow line in the formation tester. The formation tester also typically includes a pump and one or more sample chambers. Typically, fluid monitoring devices, such as optical fluid analyzers, are also inserted into the flow line to monitor the type and quality of fluid flowing at various points in the flow line.
The inlet device or probe is inserted into the formation through mudcake lining on the borehole wall. Thus, the fluid initially drawn into the flow line through the probe is a mixture of reservoir fluid and mud filtrate. To obtain a sufficiently quality fluid sample, a cleanup step in which mud filtrate is purged from the flow line is performed. This step typically involves pumping the fluid drawn into the flow line back into the borehole. However, the fluid discharged into the borehole contains reservoir fluid, which can contaminate the drilling mud in the borehole and change the properties of the drilling mud, possibly necessitating additional steps to clean or stabilize the drilling mud. As pumping continues, more and more of the reservoir fluid is consumed around the inlet of the probe. Eventually, a fluid mixture that is more representative of the reservoir fluid starts to enter the flow line. Fluid monitoring devices, such as optical fluid analyzers, are used to monitor the content of the fluid entering the flow line and how the fluid proceeds through the tool and can assist in determining when the fluid entering the flow line is of sufficient quality to be sampled.
When the mud filtrate content of the fluid entering the flow line is reduced to an acceptable level, the sample chamber is opened and fluid in the flow line is pumped into the sample chamber. Typically, the sample chamber includes a cylinder in which a piston is disposed. The sample is collected on top of the piston while the backside of the piston is exposed to either borehole pressure or atmospheric pressure. Typically, the backside of the piston is exposed to borehole pressure, which means that fluid is pumped into the sample chamber against borehole pressure. Borehole pressure is normally deliberately maintained above formation pressure to keep the well safe. Thus, pumping fluid into the sample against borehole pressure often results in the sample collected in the sample chamber being over-pressured, creating an unstable pressure-volume-temperature (PVT) environment. Moreover, in cases where a higher pressure differential is provided, additional power is typically required to pump the sample into the downhole tool.
Despite such advances in sampling technology, there remains a need to provide techniques that are capable of efficiently obtaining samples representative of the formation. It is desirable that such techniques provide pressure sufficient to prevent samples from deteriorating or becoming biphasic. It is further desirable that such techniques provide a pressure that is at a reduced pressure differential from the sample to facilitate pumping or drawing fluid into the downhole tool. Such techniques preferably provide one or more of the following, among others: maintaining sample pressure above the bubble point, reducing sampling time, reducing power requirements for sampling and balancing pressures to the formation.